Ball gags, all day every day! (I wish! lol)
I have always had a thing for the fetish aspect of a lovely red ball strapped tightly in the mouth.

The most effective gag I've experienced has been a combination of various types - namely, a few layers of duct tape wrapped over a tight bandana/scarf secured on top of a ball gag locked in place. Definitely my Mistress' favorite for when it's time for some peace and quiet ;D